The goal of plant breeding is to combine various desirable traits in a single variety or hybrid. Such desirable traits may include greater yield, resistance to diseases, insects or other pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, enhanced growth rate and improved fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same genotype. A plant cross-pollinates if pollen comes to it from a flower of a different genotype. Plants that have been self-pollinated and selected for (uniform) type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny of homozygous plants. A cross between two such homozygous plants of different lines produces a uniform population of hybrid plants that are heterozygous for many gene loci. The extent of heterozygosity in the hybrid is a function of the genetic distance between the parents. Conversely, a cross of two plants each heterozygous at a number of loci produces a segregating population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants to make hybrids, and the evaluation of the hybrids resulting from the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new plants are evaluated to determine which have commercial potential.
One valuable crop that has been subject to breeding programs is watermelon, a member of the Cucurbitacea family. The genus Citrullus originated in Africa. The plant is a large and sprawling annual, grown for its fruit. Watermelon fruit is often colored attractively, commonly red. Watermelon can contain black seeds, which are considered undesirable for certain uses.
Watermelon (Citrullus lanatus) can occur as a diploid, triploid or tetraploid. Seedless watermelon fruits are produced by using pollen from diploid male parent plants to fertilize flowers of tetraploid maternal parent plants. Pollination of the tetraploid flowers with diploid pollen leads to hybrid F1 seeds which are triploid (see, e.g., Kihara, 1951, Proceedings of American Society for Horticultural Science 58: 217-230; Eigsti 1971, Hort Science 6: 1-2). The triploid hybrid plants grown from these F1 seeds are self-infertile as they produce sterile pollen due to chromosome imbalance and need to be pollinated by a diploid pollenizer to produce watermelon fruit. Triploid plants are, therefore, interplanted with pollenizer plants for fruit production. The “seedless” fruit produced after pollination on the triploid hybrid plant are not truly seedless, but often contain some undeveloped, small, pale seeds, which are edible. Plants are generally planted at a ratio of 1 pollenizer per every 2-4 triploid plants. Triploid plants and pollenizers are either planted in separate rows (e.g., 1 row of pollenizer and 2-4 rows of triploids), or interplanted within rows (e.g., planting 1 pollenizer plant in between 2 to 3 triploid plants in the same row), or interplanted in narrow rows between rows of triploids (see e.g., Table 2 of US2006/0168701 and WO2010/111031), which is herein incorporated by reference in its entirety). The fruit produced on the pollenizer plants preferably has a different rind pattern from the fruit on the triploid hybrids, so that these can be easily distinguished.
Grading of fruits is usually done by fruit weight, to distinguish “mini” watermelons, with weights of less than 6 pounds (2.72 kg), “icebox” watermelons with weights of 8-12 pounds (3.62 kg-5.44 kg) or, according to others, of 6 to 15 pounds (2.72 kg to 6.8 kg) and “picnic” watermelons of above the icebox size, so either above 12 lb (above 5.44 kg) or above 15 pounds (above 6.8 kg). Furthermore, watermelon fruit flesh can have various colors, including various tints of red, pink, orange and yellow.
Many different watermelon cultivars have been produced, and watermelon breeding efforts have been underway in many parts of the world. Breeding objectives include varying the color, shape, texture and flavor of the fruit, absence of seeds, disease or pest resistance, optimizing flesh thickness, yield, suitability to various climatic circumstances, solid content (% dry matter), and sugar content.
Although hybrid triploid (seedless) watermelons have been grown in the United States for many decades, there is still a need for improved varieties. Consumer demand is high, and the seedless fruit of triploid watermelons are highly desired, both for the fresh and the processed markets. Many different triploid watermelon varieties exist (see, e.g., http://cucurbitbreeding.com/todd-wehner/publications/watermelon-breeding), producing fruits of different sizes and shapes, as well as different fruit quality.